"Has anyone had experience with picking a faux beam that looks real and is stainable?"

Nope, never heard of one and no pics showing.

My builder used Radiata Pine. You buy planks and make a 3 sided boxed beam. We tested different stains and came up with a mix that was a good complement to the hardwood floors. Don't forget that it will look a bit darker once up on the ceiling, so pick a slightly lighter stain than the final look you want.

You need to go to a good local lumber yard and not a big box store!

You can also buy pre-made box beams in a variety of woods, in custom lengths. If you're using a specific wood for your other accents, using the same wood is the easiest way to get a coordinating look. Just google "box beams."

Can you show us some pictures?

I follow Annie Champagne on IG and she recently completed a DIY coffered ceiling and stained the beams she built from scratch. Her IG is champagne.chaos

Beyond the material and color, the size and location of the beam(s) would be a factor in making it look real.

Not sure why my picture didn’t show up. Here’s the spot it will go. Thanks everyone! Mcarroll16 do you know where you could get the custom length ones?

Not specifically, no. It's something I looked at briefly when we were considering cladding options for an unattractive PSL beam in our kitchen. Google turned up several companies.

This would bother the hell out of me.

A real beam at the ridge of the ceiling would need proper support beneath it at certain points to carry its load and all the load it carries. The thin mullion between the narrow windows looks visually like it is not large enough to carry the load above. Even the other mullions are questionable. Check the construction drawings to see if it was built wrong or if it was designed wrong. The way it is in the picture is just another clue that the beam is fake.